1. State of the Prior Art
The present invention is related to methods and apparatus for capturing and retaining reactive chemical constituents of industrial processes that do not get reacted in thin-film semiconductor deposition reaction chambers, including, but not limited to, wet and dry scrubbers, traps, catch pots, and reactors.
2. State of the Art
Myriad thin film deposition systems, including those used for depositing thin films of a variety of semiconductor materials on substrates or devices, use selected reactive process gases to cause chemical reactions in reaction chambers that result in deposition of a desired thin film material on the substrate or device. In such systems, it is typical to feed and flow more of the reactive process gas or gases into and through the reaction chamber than actually gets reacted or consumed in the deposition process in order to maintain good thin film uniformity, so substantial amounts of the reactive process gas or gases flow out of the reaction chamber and into the exhaust system. Some of the commonly used reactive chemical process or feed gases, such as silane and other silicohydride gases, arsine, phosphine, germane, and others used in deposition systems for depositing or epitaxially growing various thin film materials are toxic, pyrophoric, or polluting, thus dangerous or undesirable in the exhaust system and/or in the atmosphere. Therefore, there has been a long recognized need for effective and efficient methods and apparatus or systems for handling such reactive chemical process gases in reaction chamber exhaust systems and for preventing them from escaping into the atmosphere.
As a particularly relevant example, silicohydride gases, e.g., silane (SiH4), disilane (Si2H6), and other silicohydride gases are highly reactive, pyrophoric, and toxic process gas chemicals used in various epitaxial growth and other deposition systems, including low pressure chemical vapor deposition (LPCVD), atmospheric pressure chemical vapor deposition (APCVD), plasma enhanced chemical vapor deposition (PECVD), hot wire deposition (HWD), and others, for depositing thin films comprising silicon, such as crystalline silicon (c-Si), polycrystalline silicon (p-Si), amorphous silicon (a-Si), silicon dioxide (SiO2), silicon nitride (SiN), and others. Silane, disilane, and other silicohydride gases are so reactive because the Si—H bond energy is low and the bonding of Si—Si chains is unstable. Typical current solutions for dealing with silane and other silicohydride process gases in the exhaust gases from such reaction chambers include various kinds of wet and dry scrubbers, which are very expensive, conventional traps and catch pots, which are only marginally helpful, or simply dumping the silicohydride process gases into house exhaust systems and expelling it to the atmosphere, which is dangerous and has undesirable pollution effects. For example, silicohydride gas exhausted into the atmosphere results in spontaneously produced micro-particles of silicon dioxide, which can be inhaled and lodged in people's lungs to cause health problems.
Both dry and wet scrubbers typically involve large and expensive capital equipment installations that discourage use, especially in countries that do not regulate effectively against exhausting silane into the atmosphere. Where regulations are not enforced or are ineffective, the availability of smaller, more efficient, and less expensive silane exhaust mitigation would make more voluntary mitigation or compliance feasible, and where such regulations are enforced, smaller, more efficient, and less expensive alternatives to conventional scrubbers would be helpful.
When a scrubber is installed to treat silane or other silicohydride gas exhausted from a low pressure chemical vapor deposition (LPCVD) process, an exhaust line is required to connect the outlet of the vacuum pump that is used to evacuate the LPCVD reaction chamber to the scrubber. For some LPCVD processes, such as silicon nitride deposition by plasma enhanced chemical vapor deposition (SiN PECVD), condensable solid byproducts are formed and will form in the exhaust line and clog the exhaust line if not prevented. Therefore, a heated exhaust line is required to keep the byproducts in the vapor phase to prevent the exhaust line from being clogged. For other processes involving large amounts of silane in the exhaust line, solid silicon dioxide (SiO2) byproducts may be formed. If water vapor is involved (which can back-stream into the exhaust line from a wet scrubber), these byproducts can stick quite strongly to the inside walls of the exhaust line, even if the exhaust line is heated. Therefore, frequent preventative maintenance, including disassembly of the exhaust line, may be required to clean out the accumulated solids before they clog the exhaust line. Also, the exhaust line may have to be very long to reach the scrubber in some installations and can be very complex, especially at the scrubber inlet, which requires not only many expensive pipe heaters, but also complex shaped, thus even more expensive, pipe heater components.
In-situ cleaning of the deposition process chamber is also quite common, where, for example, fluorine based chemicals, such as NF3, are often used to generate reactive atomic fluorine to convert solid deposition on the walls and equipment components inside the process chamber (e.g., SiO2 or Si) into gaseous compounds (e.g., SiF4) that can then be flushed out of the process chamber. To treat such halogen gases, a wet scrubber is most effective, whereas for treating silane, a dry scrubber is most effective. Therefore, either multiple scrubbers or multiple stage scrubbers with both wet and dry stages may be used to destroy different types of chemicals selectively and effectively.
As mentioned above, such conventional dry and wet scrubber systems are typically large, expensive, and not very efficient. They require a lot of space, so they cannot always be placed close to the reaction chambers and often require long runs of heated pipes, which is also expensive. There have been some attempts to provide smaller silane exhaust gas abatement, including, for example, reacting the silane with halogen to produce halosilanes that are hydrolyzed in water (U.S. Pat. No. 6,086,838, issued to Morgan), sorbent resin beds (U.S. Patent Application Publication US2005/0089455 A1 by Morganski et al.), and non-thermal, corona discharge dissociation reactors (U.S. Pat. No. 6,576,573, issued to Arno). However, more recent increases in demand for flat panel television and other displays and for solar cells have lead to large scale manufacturing of those devices. Such large scale, flat panel display and solar cell manufacturers flow significantly more silane than more conventional semiconductor processes. The current scrubber technology is ill-equipped to handle such drastically increased silicohydride exhaust gas loads, and more frequent and expensive preventative maintenance, unclogging, and cleanout cycles are now often required. Moreover, those manufacturers that, in the past, have gotten by with flowing their silicohydride laden exhaust gases directly into their house exhaust systems to avoid the capital expenditures and maintenance costs of scrubber systems are now finding that such larger gas loads are creating clogging of exhaust lines with solid silicon dioxide deposits and safety and environmental hazards.